| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 105 === In this thesis, the behavior of near-field thermal radiation generated from the coupling between magnetic dipole resonance and evanescent wave are investigated. Generally the extinction coefficients of the polar materials and the metallic materials are greater than their refractive indices in the middle infrared band. That is, the real part of the dielectric constants of the polar and metallic materials is negative. At this condition, the surface wave mode would be supported on the boundary between air and polar materials (or metallic materials). The presence of surface waves is accompanied with evanescent wave, an electric field decay rapidly along the normal direction of surface. Though an evanescent wave cannot deliver energy directly, the energy could be coupled to nearby objects within sub-wavelength gap. Previous studies used polar materials based superstrates or structures having surface mode to couple with the evanescent wave. Furthermore, these studies illustrated that the energy transfer between two objects could be larger than the black body radiation when the gap between two objects was very close. However, these method cannot expel the thermal energy to surrounding through radiation.
In this thesis, the magnetic dipole resonance of silicon particles was used as a coupling medium of surface waves. The thermal energy can further emit to environment through resonant scattering, induced by magnetic dipole resonance, and further achieving emissivity enhancement.
Silicon carbide and aluminum both can support surface wave mode. The first part of this thesis discuss the silicon particle affecting the emissivity of the silicon carbide. The second part changed the substrate material from silicon carbide to aluminum. We used three dimensional finite difference time domain (3D-FDTD) method to simulate the influence of radiation when the silicon particles put on the silicon carbide substrate or aluminum film.
In the experimental part, we heated the samples with/without silicon particle to the 70 oC, then measured the cooling time to compare the difference of the emissivity. In the study of silicon carbide substrate, the cooling time of a bare silicon carbide substrate, a silicon carbide substrate with surface coverity 0.845% of silicon particles and a silicon carbide substrate with surface coverity 2.205% of silicon particles were 3203, 1237, and 1104 seconds, respectively. In the study of aluminum film, the cooling time of a bare aluminum film, an aluminum film with surface coverity 2.19% of silicon particles and an aluminum film with surface coverity 5.72% of silicon particles were 4895, 2787, 1432 seconds, respectively. The huge influence of the low surface coverage of silicon particles on the thermal emission of silicon carbide substrate or aluminum film were observed.
In addition, in order to confirm that the radiative heat transfer between the silicon particles and the silicon carbide or aluminum film substrate resulted from the near-field thermal radiation, we capped the silicon carbide substrate with a silicon superstrate coated with few silicon particles. Four photoresist pillars were made on the silicon superstrate to control the gap size between superstrate and silicon carbide or aluminum film substrate. When the height of pillars were 3.1, 4.0, 5.2 and 5.9 microns, the cooling time of silicon carbide substrate from 70 oC to the equilibrium room temperature were 1178,1569,1778, and 1937 seconds, respectively. The heat transfer reduced rapidly when the height of pillars was increased. Moreover, we recorded the average emissivity with temperature by infrared camera. At 180 oC, the emissivity of bare silicon carbide was 0.648 and the emissivity of silicon carbide coated surface coverity 2.205% of silicon particles was increased to 0.685. The emissivity enhancement is obviously greater than the surface coverage of silicon particle. When the substrate was replaced by the aluminum film, the emissivity of bare aluminum film was 0.03 and the emissivity of aluminum film with 5.72% silicon particle coverity was increased to 0.05. It’s surprising that emissivity enhancement up to 60% at low coverity of silicon particles. The experimental results revealed that the magnetic dipole resonance of silicon particles could effectively couple with surface wave and scatter electromagnetic energy to the surrounding.
In this study, we demonstrated that silicon particles could couple effectively energy with surface wave of polar materials or metallic film within the near field regime. When the energy of surface waves coupled into the silicon particles, the magnetic dipole resonance effect could further scatter the surface wave to far-field. Though the size and shape of silicon particles are not uniform, the phenomenon of thermal emission enhancement is still obvious. Therefore the technique has great potential to apply on advanced cooling engineering in the future.
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